Quantum entanglement shows that reality can’t be local

Either that, or faster-than-light communications is a go.

Quantum entanglement stands as one of the strangest and hardest concepts to understand in physics. Two or more particles can interact in a specific ways that leave them entangled, such that a later measurement on one system identifies what the outcome of a similar measurement on the second system—no matter how far they are separated in space.

Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement: there's no slower-than-light influence that can pass between the entangled particles. However, one possible explanation for entanglement would allow for a faster-than-light exchange from one particle to the other. Odd as it might seem, this still doesn't violate relativity, since the only thing exchanged is the internal quantum state—no external information is passed.

But a new analysis by J-D. Bancal, S. Pironio, A. Acín, Y-C. Liang, V. Scarani, and N. Gisin shows that any such explanation would inevitably open the door to faster-than-light communication. In other words, quantum entanglement cannot involve the passage of information—even hidden, internal information, inaccessible to experiment—at any velocity, without also allowing for other types of interactions that violate relativity.

Experiments have definitively demonstrated entanglement, and ruled out any kind of slower-than-light communication between two separated objects. The standard explanation for this behavior involves what's called nonlocality: the idea that the two objects are actually still a single quantum system, even though they may be far apart. That idea is uncomfortable to many people (including most famously Albert Einstein), but it preserves the principle of relativity, which states in part that no information can travel faster than light.

To get around nonlocality, several ideas have been proposed over the decades. Many of these fall into the category of hidden variables, wherein quantum systems have physical properties (beyond the standard quantities like position, momentum, and spin) that are not directly accessible to experiment. In entangled systems, the hidden variables could be responsible for transferring state information from one particle to the other, producing measurements that appear coordinated. Since these hidden variables are not accessible to experimenters, they can't be used for communication. Relativity is preserved.

Hidden variable theories involving slower-than-light transfer of state information are already ruled out by the experiments that exclude more ordinary communication. Some modern variations combine hidden variables with full nonlocality, allowing for instantaneous transfer of internal state information. But could non-instantaneous, faster-than-light hidden variables theories still work?

To investigate this possibility, the authors of the new study considered the possible experimental consequences. Obviously, one way to test it would be to increase the separation between the parts of the entangled system to see if we can detect a delay in apparently instantaneous correlation we currently observe. Sufficiently fast rates of transfer, however, would still be indistinguishable from nonlocality, given that real lab measurements take finite time to perform (this assumes that both experiments happen on Earth).

The researchers took a theoretical approach instead, using something known as the no-signalling conditions. They considered an entangled system with a set of independent physical attributes, some observable, some hidden variables. Next, they allowed the state of the hidden variables to propagate faster than the speed of light, which let them influence the measurements on the separated pieces of the experiment.

However, because of the nature of quantum mechanical systems, there was a symmetry between the hidden and measurable attributes of the system—meaning if the hidden variables could transfer information faster than light, then the properties we can measure would do so as well. This is a violation of the no-signalling condition, and causes serious problems for the ordinary interpretations of quantum physics.

Of course, one conceivable conclusion would be that faster-than-light communication is possible; this result provided a possible avenue for testing that possibility. By restricting the bounds on the speed of interaction between entangled systems, future experiments could show whether any actual information is traveling or not.

However, the far more likely option is that relativity is correct. In that case, the strong ban on faster-than-light communication would rule out the possibility of faster-than-light transfer of information encoded in hidden variables, and force us to deal with nonlocality. Once again, it would seem that local realism and relativity are incompatible notions in the quantum world.

Thank you for this article! as always they're informative and thought provoking.

When I read any article about quantum entanglement I'm always looking for a reason that a simple-minded explanation of the phenomenon is ridiculous: all objects entangled are the same object; and would be seen to be so if one could view down the 'correct' dimensional projection. Think of the flatland occupant that sees a single object as being two because it enters the 2d plane at two separate places.

This has to be wrong because it's too simple and therefore would have been immediately discounted - but why not a single mention of it (even in off-hand dismissal)?

What if the two entangled particles were points on a plane or object in, for lack of a better term, the fourth dimension? Is it possible entanglement is an analog to a Flatlander observing a cone or sphere passing through 2D space?

I don't know much about physics. That being said, why can't it just be as follows:

Analogy: take two boxes and put the same message in each. Separate the boxes a million light years. The instant you open each box, you will get the same message at each location, without FTL travel.

These aren't hidden variables necessarily, just variables which are set to the same values. Or maybe I'm misunderstanding, and these are hidden variables...

That was my understanding as well. Except the values are asymmetrical.

Except that entanglement isn't actually about getting a message at all. Following the message in a box example... imagine that they are perfectly preserved, then one box gets opened. It undergoes a change as it begins deteriorating. Simultaneously without the other box also being opened, the other message a million light years away, also begins deteriorating. When you look at the second message, you instantly know that the other message has been read because it has already begun deteriorating before you opened the box.

In the quantum world you don't actually know what the original state is. You can only observe the change in state. That is you know when the object changes, but you don't know what caused the change. Only by communicating by conventional means can you determine what caused it.

Of course you might assume that this means you can watch for a change and thus gain information in a simple system where change = 1 and no change = 0. Problem is that the change isn't necessarily caused by the entanglement. You have to verify.

It is much more elegant for the Bell theorem to come down against local reality, if you ask me. So I like this result.

With entanglement, you have a wavefunction that you stretch without causing it to decohere. You can decohere the whole thing at once because there is just one wavefunction. But that isn't an information transfer.

I don't know much about physics. That being said, why can't it just be as follows:

Analogy: take two boxes and put the same message in each. Separate the boxes a million light years. The instant you open each box, you will get the same message at each location, without FTL travel.

These aren't hidden variables necessarily, just variables which are set to the same values. Or maybe I'm misunderstanding, and these are hidden variables...

From what very little I understand of quantum entanglement, the visible "entangled" properties are too random to reliably transmit any information, i.e. it's just jibberish.

Then again I'm probably wrong about that (in my current quantum state).

Not exactly. Entanglement means that when one particle's state is measured, the state of the other particle is set (instantaneous or FTL is still unknown, as the article states). However, we cannot influence what that state will be (which is what is required to actually transmit information), so there's no way to send a signal via that mechanism.

Here's an example. Imagine the entangled particles are a set of two six-sided dice, where the side that lands up is always the opposite of the other die - but they can only be rolled once (as is the case for any entangled set of particles - the act of measurement breaks the entanglement). Take them and separate them such that any communication between them would be FTL. I can roll a die and get a 6, and instantly know that the other will be a 1 - but now that I've rolled the dice, I've broken the entanglement and I can't further turn my die to force the other to change its up-face as well.

To the article author/editors:Can you add the Technopaedia entry on entanglement (I think I've seen one before) to articles like these?

The "each particle carries a message" interpretation is the local hidden variable interpretation favored by Einstein et al, which is disproved by Bell's Inequality and the experiments that tested it (the reason this is nonintuitive given the "stick a message in a box" analogy is that Bell's Inequality has a subtle, very quantum-y derivation). The "global hidden variable" interpretation or "quantum mechanics is literally true" are the only ones left standing, both of which have nonlocality. The first is nonlocal but deterministic in some sense, the latter is nonlocal and purely based on probability distributions. This result, if I'm reading it correctly, places limits on what forms the mechanism of nonlocality in either case could take.

I don't know much about physics. That being said, why can't it just be as follows:

Analogy: take two boxes and put the same message in each. Separate the boxes a million light years. The instant you open each box, you will get the same message at each location, without FTL travel.

These aren't hidden variables necessarily, just variables which are set to the same values. Or maybe I'm misunderstanding, and these are hidden variables...

You have to use cats instead of messages, together with something that may or may not kill the cat [physics is a cruel science]. Then you won't know the contents of one box until you open it, and, because the boxes (or was it the cats?) are entangled, the result of opening the other box will be the same.

I'm really surprised that no-one as mentioned many worlds so far. If many worlds is true, then quantum entanglement does not violate locality.

Many worlds is disgusting in its own way. Think about it: every single outcome possible under quantum mechanics, no matter how unlikely, is realized. There exists a Universe where, if it's Tuesday and you look up and to the left, every radioactive atom in your body simultaneously decays coupled with the fusion via very low probability tunneling of every hydrogen atom in your spleen, resulting in a massive thermonuclear explosion. That Universe is out there somewhere in Many Worlds, and it's no less likely than the exact one we live in (though the one we live in tends to look very similar to the average case, and let's hope it stays that way).

I don't know much about physics. That being said, why can't it just be as follows:

Analogy: take two boxes and put the same message in each. Separate the boxes a million light years. The instant you open each box, you will get the same message at each location, without FTL travel.

These aren't hidden variables necessarily, just variables which are set to the same values. Or maybe I'm misunderstanding, and these are hidden variables...

From what very little I understand of quantum entanglement, the visible "entangled" properties are too random to reliably transmit any information, i.e. it's just jibberish.

Then again I'm probably wrong about that (in my current quantum state).

Not exactly. Entanglement means that when one particle's state is measured, the state of the other particle is set (instantaneous or FTL is still unknown, as the article states). However, we cannot influence what that state will be (which is what is required to actually transmit information), so there's no way to send a signal via that mechanism.

Here's an example. Imagine the entangled particles are a set of two six-sided dice, where the side that lands up is always the opposite of the other die - but they can only be rolled once (as is the case for any entangled set of particles - the act of measurement breaks the entanglement). Take them and separate them such that any communication between them would be FTL. I can roll a die and get a 6, and instantly know that the other will be a 1 - but now that I've rolled the dice, I've broken the entanglement and I can't further turn my die to force the other to change its up-face as well.

To the article author/editors:Can you add the Technopaedia entry on entanglement (I think I've seen one before) to articles like these?

Pardon my ignorance (and picking on your post in particular):

What's to stop me from sending multiple dice, each in their own labeled boxes?

The holographic theory of the universe covers this aptly. If the universe is a giant hologram then our perception of distance is actually an illusion. Therefore two entangled particles would actually be different perspectives on the same part of the hologram and thus not separate at all.

Wait, I'm a little confused. Fundamentally, locality states that an object is influenced directly only by its immediate surroundings (usually seen through some particles transmitting forces in the Standard Model from one physical body to another). So if you reject that, you can have something changing something else at a distance, without the use of some intervening system of particles, waves, or other form of medium. So far, so good (sort of).

But does that necessarily mean that faster-than-light communication is possible? If by manipulating A I can change B at some arbitrary distance through no intervening force, particle, or other object of any kind, then I can cause an effect that travels faster than light. And that contradicts relativity right there. Einstein formulated relativity the way he did because he believed in locality. If you throw out locality, doesn't relativity (at least, the part that assumes FTL communication is impossible) go along with it?

An interesting note is that, up until quantum mechanics came on the scene, every scientist did just about everything possible to avoid breaking locality. Everyone held that it was inviolable (Newton even going so far as to say it was impossible for any man competent in rational thinking to hold nonlocality as possible). I, for one, think it far more likely that FTL communication is possible. But, I've been wrong before, once or twice.

Physicists need to read "Flatland" by Edwin Abbott Abbott. Read and understand how the two dimensional beings freak out whenever a three dimension object passes though their reality.

A three dimensional ring passing though a two dimensional world (like a wedding ring passing though a piece of paper perpendicularly) appears at first to be one particle then splits into two (and these two "particle" appear to have no connection in the two dimensional world) yet tap one and the other shakes (the particle are "spookly" entangled to the two dimensional beings), then the two particles recombine and disappear.

Quantum entanglement looks exactly like an object in in another dimension passing through our "normal" dimensions. Everything from spooky non-locality to spontaneous particles jumping into and out of reality, is easy to understand if an extra "quantum" dimension is imagined.

The holographic theory of the universe covers this aptly. If the universe is a giant hologram then our perception of distance is actually an illusion. Therefore two entangled particles would actually be different perspectives on the same part of the hologram and thus not separate at all.

Holographic theory is not perception of distance being an illusion, it's about three dimensions of space being equivalent to two dimensions of space, with neither really being different. So the so-called particles (what will be particles when they decohere) are still separate, in that the wavefunction can be interacted with in two spatially separated locations.

I don't know much about physics. That being said, why can't it just be as follows:

Analogy: take two boxes and put the same message in each. Separate the boxes a million light years. The instant you open each box, you will get the same message at each location, without FTL travel.

These aren't hidden variables necessarily, just variables which are set to the same values. Or maybe I'm misunderstanding, and these are hidden variables...

Hm... I think its kind of like if each box had a magic 8 ball suspended in it. They go a million light years away, you shake a box, open both, they read the same message. *cue twilight zone music*

Only the suspension of the 8-ball is a mental construct, a placeholder to that we use to say “it could be anything” until we open the box. So in this analogy entanglement is the synchronizing of two 8-balls to the same answer without leaking that information to anything other than the two 8-balls. Thus the spooky action at a distance is revealed as a misunderstood parlor trick.

What I don't get is why physicists treat entanglement and measurement as two different things. When two, or more, particles interact, they measure each other. Careful control by us of that interaction creates what is called entanglement because they have yet to interact with or be measured by any other particles.

They are logically identical processes, except for the number of interactions involved. So why can't a measurement be seen as simply entangling ourselves with the particles being measured?

What's to stop me from sending multiple dice, each in their own labeled boxes?

i.e. "Attack" "Report" "Return"

It'd be one way communication, but marching orders all the same.

If you're looking for a particular option to be chosen from among the orders (say, by doing whichever box contained a die with a 1 up-face), there's no way for me to set the die value on my end (I can only roll the die), so you'd be more or less choosing a random set of orders.

No problem with picking on my post - feedback is always important when coming up with analogies .